The hormone, prolactin, is involved in the development and hormonal differentiation of the mammary gland. In addition, prolactin has been implicated in the growth of mammary tumors. The objective of this study is to delineate the interaction of prolactin with its receptor in order to define the biosites in prolactin and the receptor which are responsible for the mechanism of action of this hormone. The prolactin receptor has been purified to homogeneity and with good yield in this laboratory. Sufficient amounts of receptor from rabbit mammary gland will be isolated and characterized. The amino acid sequence from the N-terminal end will be obtained by gas phase sequencing, and oligonucleotide hybridization probes will be synthesized to be used for identifying clones of the prolactin receptor. The DNA sequence determination will provide the amino acid sequence and the receptor will be expressed in bacteria. The gene for ovine prolactin will be cloned, expressed in bacteria, and the dNA sequence will be determined. Oligonucleotide mutagenesis will be used to alter selected amino acids in both prolactin and the receptor in order to determine their biosites.